Soybeans are, primarily, an industrial crop, cultivated for oil and protein. Despite the relatively low oil content of the seed (about 20% on moisture-free basis), soybeans are the largest single source of edible oil and account for roughly 50% of the total oilseed production of the world. With each ton of crude soybean oil, approximately 4.5 tons of soybean meal with a protein content of about 44% is produced. For each ton of soybeans processed, the commercial value of the meal obtained usually exceeds that of the oil. Thus, soybean meal is much more than just a by-product of the oil manufacture. The bulk of soybean meal is used in animal feeds.
However, several of the proteins naturally found in soybeans have been found to exert specific physiological effects, such as inhibition of the proteases in the digestive tract of animals and humans. Soybeans contain two types of so-called trypsin inhibitors. They are respectively known as the Kunitz trypsin inhibitor with a molecular weight in the range of 20,000, and the Bowman-Birk inhibitor which is a much smaller polypeptide in the 8,000 dalton range. Both types consist of a number of differentiable proteins. The amino acid sequence and spatial structure of these proteins have been elucidated.
Ingestion of trypsin inhibitors have several physiological effects, including inhibition of trypsin followed by increased pancreatic secretion. This leads to internal loss of protein to the digestive tube as well as hypertrophy of the pancreas. Therefore, inactivation of these proteins to improve digestibility is highly desirable.
The activity of trypsin inhibitors can be reduced by heat treatment of the soybeans. Such heating, however, may also destroy or reduce the availability of certain heat sensitive amino acids and reduce the nutritional value of soy protein. Also, when proteins are heated in the presence of certain carbohydrates, the sugars will complex with free amino groups resulting in a series of reactions called the Maillard reaction, which is for several reasons not desirable. Therefore, other approaches to reduce the activity of trypsin inhibitors and thus improve soybean nutritional values are being searched for.
In WO01/58276, proteolytic degradation of pure Bowman-Birk and Kunitz trypsin inhibitors was demonstrated. In WO98/56260 and WO03/041510, proteolytic treatment of soy protein material to degrade or deactivate trypsin inhibitors is described. The proteolytic enzymes exemplified in these publications are rather unspecific, and WO98/56260 mentions broad specificity as one of the most important parameters for a proteolytic enzyme to be able to degrade such antinutritional factors.
U.S. Pat. No. 4,512,973 showed inactivation of soy trypsin inhibitor using starfish trypsin 1 and a supplementary proteolytic enzyme. It was shown that hydrolysis with starfish trypsin alone did not inactivate the soy trypsin inhibitor. At least one additional proteolytic enzyme, such as carboxypeptidase B, was needed for inactivation.
One purpose for the present inventors has been to provide new methods for inactivation of soy trypsin inhibitors. Inactivation without excessive heat treatment provides distinct nutritional advantages leading directly to economic benefits.